Fenix 21700 is a rechargeable battery with a capacity of 5000 mAh. For comparison, this is 50% more battery capacity than a regular smartphone. However, you don't need a charger to charge it. A simple USB-C cable will suffice. You can charge the battery either from the mains or from your computer. The Fenix 21700 boasts overheat protection and a special anti-blast valve. In addition, it also has overcharge, full discharge and short circuit protection and a protection circuit on the plus pole. This makes it one of the safest Li-ion batteries on the market. Moreover, it does not suffer from memory effect or discharge.
Unfortunately it is not compatible with extended 18650 batteries, but it still has a wide range of uses. For example, you can use it in Fenix HT18, Fenix PD36R, Fenix TK22 V2.0 or Fenix TK22 UE flashlights.